The present invention relates to means for testing transmissive optic fibers, and more particularly to measuring dispersion produced by a single mode fiber.
Single mode optical fibers are used to transmit large quantities of information and are capable of transmitting optical information over significant distances. As with any other medium of transmission, it is necessary that optical fibers transmit the information carried thereby with fidelity. In the real world, various forms of distortion introduced by transmissive media cannot be eliminated. It is, therefore, necessary to measure them, either to determine the suitability of a transmissive medium or to evaluate the manner in which such distortion will be dealt with. For a fiber optic communications system, the most significant specification for determining the information carrying capacity of the system is the bit-error rate. The bit-error rate is increased, among other factors, by the pulse broadening caused by dispersion in a fiber. Use of a single mode fiber eliminates modal dispersion. Chromatic dispersion will still remain as an inherent characteristic of a fiber. Fibers will relay different wavelength radiation differently. It has further been noted in accordance with the present invention that not only chromatic dispersion but polarization dispersion can be present as the contributor to the bit-error rate.
It is generally impractical to conduct a bandwidth measurement on a single mode fiber to determine its information carrying capacity due to the high bandwidths of single mode fibers. It is more useful to measure chromatic dispersion. Chromatic dispersion occurs due to material dispersion, the difference in index of refraction versus wavelength of a transmissive medium and also to waveguide dispersion. In a standard doped silica fiber, material dispersion varies monotonically and passes through zero in the vicinity of 1300 nm. Waveguide dispersion as a source of dispersion increases with increasing wavelength. In a known standard doped silica fiber, the contribution of waveguide dispersion is small where material dispersion is zero and the zero dispersion wavelength inherently falls in the range of 1300 nm, a naturally occuring low attenuation region. By controlling the waveguide dispersion through complex geometry of the fiber, the zero dispersion wavelength of the fiber can be forced to fall in the region of 1550 nm where the attenuation of a doped silica based fiber is even lower than at 1300 nm. Chromatic dispersion measurements are used to establish how well a particular fiber is matched to a particular transmission wavelength.
Many prior art techniques exist to measure chromatic dispersion. See, for example, National Bureau of Standards Symposium on Optical Fiber Measurements, NBS SP683, 1984. The most common technique measures propagation times of short pulses with respect to wavelengths. Such techniques are limited in resolution due to the necessity of recovering of extremely short optical pulses and due to inherent stability problems with pulse timing circuitry. Chromatic dispersion can also be measured by determining the phase shift of a continuous wave modulated light beam versus wavelength. In this prior art technique, a light source is amplitude modulated by a high frequency generator and passed through a monochromator for transmission through a fiber under test. The output of the test fiber is sensed by a photodiode, such as a germanium or galium arsenide photodiode which provides an output signal phase displaced from the signal entering the fiber. The difference in phase of the reference and the output signal is measured by a time difference detector such as a phase detector. After phase shift is determined for the fiber at one wavelength, the monochromator is changed and the entire process is repeated at another wavelength. This process in effect is a process in which a relative delay curve is generated. In order to compute dispersion a first derivative is taken, namely change in phase delay with respect to wavelength. The mathematical process of differentiation is undesirable since it also increases the uncertainty of the measurement and exacerbates noise. Due to inherent drift over relatively short periods of time of system components, uncertainty arises as to whether the difference in phase is due to chromatic dispersion or to drift. Thus measurement is subject to an effect called 1/f noise.